What is the difference between a TT and an ETT? And what makes the PS decide which procedure to perform?

It simply has to do with the amount of skin laxity the patient has, and the tailoring needed to correct it.  Someone with more laxity, especially towards the sides of the abdomen, is more likely to need an extended approach.

Thank you for sharing your excellent question.  In general the difference between the two operations is the length of the scar.  The extended tummy tuck has a longer horizontal scar component because of the desire to directly excise loose tissue found around the love handle and waist region.  Hope this helps.